Answers to the questions most often asked about sailboat refrigeration.

Do I want a 12 volt DC refrigerator? I am a sailor and never run my engine.
How many amps does it draw?
How much will it run? Does your BD compressor speed up when the engine is running?
Does your BD compressor speed up when the engine is running?
Isn’t a 12 volt DC system with a holding plate the best?
How big should my battery bank be?
Is water cooling better?
What is keel cooling?
What is the advantage of the DC 5000 system?
Can I power with solar and wind?
Can I leave my boat for a week unattended?
Why can’t I have a one plate freezer?
I want to sail. Why should I consider an engine drive?
What system cools the fastest?
What is the best way to improve efficiency?
Why do I need to insulate the top of my ice box? 
Why does the bulkhead next to our refrigerator sweat?
What is vacuum insulation? 
Can I make ice cubes with a refrigerator or do I need a freezer?
How fast can I run the engine compressor on the Engine Drive?

Q: Do I want a 12 volt DC refrigerator? I am a sailor and never run my engine.  
A: Remember that batteries are an energy storage system not an energy source. Without lots of alternate energy from a wind charger and solar panels you will have to run your engine or generator to replace the energy taken from your batteries. Installing a 12 volt refrigeration system requires understanding how much power you will need and how to store and replenish it.

Q: How many amps does it draw?
A: The answer has no value in choosing a system without knowing how much work is being done. The common small compressor now used by most manufactures is the Danfoss BD 50. The BD 50 can be set to operate at different speeds. At slower speeds the draw can be as low as 2.5 amps/hr at 12 volts. If the compressor is set at high speed it may draw 5 amp/hr at 12 volts. At the high speed verses the low speed twice the work is done. A unit drawing 2.5 amp/hr will run twice as long to do the same work using the same total amp/hrs per day. The physics of heat movement in refrigeration applications allows about 3.5 BTUs per watt in tropical duty.

Q: How much will it run?
A: Compressor running time is based on the size of the box being cooled, the type and thickness of insulation, the desired temperature, the climate, and the amount of food being cooled. The average power consumption for production boats is between 40 to 90 amp/hrs. per day for refrigerators in tropical waters.

Q: Does your BD compressor speed up when the engine is running?
A: No. The BD compressor is set at high speed to obtain the fastest cooling rate. Because the compressor is so small the difference between low and high has minimal impact on the batteries (2.5 or 5 amp/hrs at 12 volts) and, even at high speed the cooling rate is not rapid. The advantage to a changing the compressor speed and slowing it down is to more carefully match the compressor to small cabinets.

Q: Isn’t a 12 volt DC system with a holding plate the best?
A: Holding plates were invented to capture thermal energy when it is available using a big compressor. Due to the compressor size a 12 volt holding plate system with a BD compressor as a primary system is redundant and inefficient. Draining a battery to freeze a holding plate is an inefficient swap of energy. Another problem is created by connecting a holding plate to a small compressor system as it interferes with the heat transfer between the refrigerant and the air in the cabinet. The holding plate becomes an insulator as a layer of ice forms on the refrigerant tubes inside the plate, slowing the cooling. This is like operating an old style non-defrosting home refrigerator with heavy ice on the cooling section. The compressor keeps running but the effective cooling is reduced. In some applications small compressors are not capable of freezing a holding plate because their cooling rate is just a bit more than the cooling required. In this situation the desired compressor off time the holding plate is intended to produce is never possible. For a proper 12 volt holding plate system the high draw ½ hp DC 5000 should be installed to freeze plates rapidly. It is 10 times more powerful (faster) than a BD compressor.

Q: How big should my battery bank be?
A: You can’t have too many batteries. Rule of thumb says only one third of a batteries’ capacity (20 hr rating) can be used while cruising. Look at your refrigeration requirements and also the other 12 volt devices. If in your calculation for 24 hours you are using more that one third of your capacity, turn off some equipment or get more batteries. Excessive battery discharge will wear out your batteries rapidly. The larger your bank is the faster it will accept a charge and the longer it will last.

Q: Is water cooling better?
A: Water has a high density and absorbs heat quickly. Using water in a large system makes perfect sense. In a small system where the cooling rate is low and constant, water cooling is less an advantage. The optional water cooling on our BD unit increases the amp/hr draw by .7 amp/hr due to pump draw. It also requires a through hull, strainer and discharge through hull. As a maintenance issue the water pump needs to be replaced after about one year of constant use. The standard BD unit is designed with a double sized air condenser and features ducted air cooling. The fan in the BD uses less that .33 of an amp to supply ambient air. Ducted air cooling is simple, quiet, reliable and works well in the tropics.  Water cooling should be considered if the compressor must be installed in a hot engine space or an area that can't be ducted.

Q: What is keel cooling? 
A: A keel cooler is a metal heat sink on the outside of the hull, below the water, that refrigerant passes through to dissipate the heat being removed from the refrigerator. It requires drilling a hole in the hull or replacing a through hull. Keel coolers are subject to fouling, and electrolysis. Fouling reduces the heat transfer as does stagnant water. Keel coolers are the most effective when the boat is moving and the cooler is clean. Keel cooling does not require a fan or pump. In still warm water, when there is no scrubbing effect of water motion, keel coolers can make the compressor work extremely hard without the benefit of forced air cooling of the compressor and it’s electronic control.  Click here to see test data on keel cooling versus air and water cooling.

Q: What is the advantage of the DC 5000 system?
A: This is a high capacity compressor system that may be used with large refrigerators and freezers. The DC 5000 can freeze holding plates, operating when the generator or main engine is on, as well as operating from a large battery bank. Unlike the BG 1000 or Engine Drive, which require scheduled engine operation, DC systems give some flexibility in the battery recharging schedule. A DC 5000 system with a properly sized battery bank can operate on demand from thermostats and maintain even cabinet temperatures. The DC 5000 ½ HP 12 volt unit will draw about 40 amps/hr. The electrical efficiency is 4 to 6 BTU per watt in tropical climates.

Q: Can I power with solar and wind?
A: Sailors in the Bahamas using winter winds can break even on their refrigeration load if the refrigerator box is small and well insulated. Adding solar panels will help but this may require many square feet of area. Remember lighting, computers, stereos, water makers, pumps and electronics have to be factored into the daily loads.

Q: Can I leave my boat for a week unattended?
A: Batteries, batteries, batteries. Calculate your daily amp/hr. refrigeration requirement. Estimate the added amp/hrs from solar panels and wind generators based on your equipment and your judgment of the weather. Add the seven day contribution of sun and wind to 1/3 of your 20 hour battery rating. Divide your refrigeration requirement into your available amp/hrs and get at least 7. All this is possible but rarely practical.

Q: Why can’t I have a one plate freezer?  
A: A one plate freezer is always a problem if air flow is restricted. Air is cooled by the cold plate and must then travel to the frozen food being stored. Most freezers are jammed full of food restricting air flow. A classic analogy is to stand in front of a heater, you will feel warm in front and your back will be cool. Stand between two heaters and feel quite hot. This of course is the reverse of our desired operation but the concept of two sources of energy is the same. A two plate freezer benefits from more plate area to air volume which produces a closer plate to box temperature. A two plate system will suffer less from ice and frost build up on the plates which slows the cooling process. The sandwich effect of two plates on either side of the freezer cools the middle with little air flow.

Q: I want to sail. Why should I consider an engine drive?
A: The Engine Drive is a sailor’s system. The refrigeration load is eliminated from the batteries. Cold plates can be frozen rapidly with a large compressor turned by the inboard auxiliary. In most cases this process is faster than batteries can take a charge. Properly sized the Engine Drive is only run once a day for an hour. The Engine Drive is mechanically the simplest system and stands alone. It does not require the support of a secondary system of batteries and charging equipment, a good insulated cabinet is all that is needed for excellent performance. An Engine Drive can be augmented with a second compressor system for 110 volt dockside, 110 volt generator, or 12 volt DC for alternate energy or if surplus 12 volt DC is available.

Q: What system cools the fastest? 
A: Holding plate systems were conceived to store energy as frozen material. Large compressors and efficient refrigeration components are used to freeze the holding plates as fast as possible. The fastest cooling (shortest running times) comes from an engine driven compressor because it can use several horsepower from the engine.

Q: What is the best way to improve efficiency?
A: A refrigeration system is a pump removing heat that leaks into an insulated cabinet. The pumping/cooling rate must be greater that the heat leak, in order to make the cabinet cold. Insulation controls the rate heat leaks in. Proper insulation means less pumping and possibly a smaller pump. Be sure the cabinet insulation is good for the best performance.

Q: Why do I need to insulate the top of my ice box?
A: Cold air does sink but heat travels in all directions. An un-insulated lid that is warm on the top will be warm on it’s under side and that’s heat in the box!

Q: Why does the bulkhead next to our refrigerator sweat?
A: Sweating is moisture in the air that has condensed to water on a cool surface. Moisture on a lid or wall is an indication that the insulation is too thin and that the refrigerator is chilling through the insulation.

Q: What is vacuum insulation?  
A: Vacuum panels or vacuum insulation is a "high tech" material in an air tight wrapping that has been evacuated of all air. Since heat can travel by air movement, removing the air with a vacuum pump reduces the conductivity of the panel. Claims are impressive with a one inch thick panel replacing up to 4 inches of polyurethane closed cell foam. The drawbacks are many; each flat area must be made as a custom panel, the panel will not bend, the cover material is conductive, for this reason the corners and intersections of panels have essentially no insulation. Moisture in the form of humid air will condense at the corners and intersections. Moisture must not be allowed to penetrate the corners and intersections, frost and ice can damage the construction. Vacuum can be lost by damage (drilling) as well as aging. A panel that looses its’ vacuum will drop in insulation value to about the equivalent thickness of foam. Panels have an unknown life span and most likely will need to be replaced during the life of the boat. Servicing a failed panel could mean totally rebuilding counter tops and furniture.

Q: Can I make ice cubes with a refrigerator or do I need a freezer?
A: Water needs to be frozen to make ice. A refrigerator plate drops below freezing to cool a refrigerator. Any of our systems will make ice cubes with ice trays in contact with the cold plate.

Q: How fast can I run the engine compressor on the Engine Drive?
A: A properly installed system may be operated at any engine speed. When anchored operating at 1200 RPM is sufficient speed for rapid freezing.